Connecting to down-hole installed equipment, such as a pressure sensor and/or a temperature sensor or else to a pump, via a cable such as an electrical cable is now common in the oil business. The use of electric submersible-pump power cables and the attachment of instrumentation cables to down-hole devices have been known for many years, especially on land and in shallow water.
The sub-sea environment (operations where the oil well is effectively constructed with its datum and attached pipe-work at seafloor level) presents special challenges for engineers. A sub-sea operation that could straightforwardly be undertaken on dry land has to be undertaken with specialist equipment that has failsafe modes and appropriate margins for failure of equipment. Even with the use of divers and ROVs (remotely operated vehicles), certain operations cannot be undertaken at sea floor level.
During well construction, water depth usually precludes the use of fixed work platforms secured to the seabed. Instead, semi-floating work platforms (semi-submersible rigs) are floated out to the work area and either secured by chains or kept on station by satellite co-ordinated thrusters (i.e. the platforms are dynamically positioned).
Since the well equipment is located on the seabed, whilst being suspended from the semi-floating platform, it is difficult to attach cables to the equipment. There is also a risk that any electrical cable or delicate equipment could easily be damaged during the installation procedure.
Over the years the number of pockets of known hydrocarbon deposits that are accessible by land has diminished, and even those deposits that are accessible within shallow water are becoming scarce. Consequently, operators are moving into ever greater water depths to access oil reserves. This has led to a requirement for more complex, time consuming and costly operations to access and produce oil in deep water. At the same time, the necessary technology to monitor down-hole conditions has become more freely available. What was originally all mechanical equipment is now frequently being replaced by a combination of mechanical and sophisticated electronic monitoring equipment to optimise and monitor well conditions. Whilst the technology to develop electronic sensors and equipment robust enough to work in the harsh sub-sea environment is now available, the methods of connecting and switching the signals are still under development.
As outlined above, there is a drive towards drilling in deeper, more remote waters and to monitor well conditions and performance in order to optimise return on investment. This has led to a review of operations previously considered as routine in order to save the significant increased costs of these operations or the cost of their failure in the deepwater environment. For example, the operation of installing tubular production strings (conduits for the oil) and connecting a permanent monitoring cable to a down-hole device might now take much longer on deep sub-sea wells. Previously, if the equipment was installed without cable or sensor monitoring and it was found to have failed, the equipment would be pulled back out (a so-called “work over”) and the damaged item repaired. However, in the deepwater environment, these work over (repair) costs are becoming prohibitively high.
One method for monitoring and therefore controlling the well after installation requires the use of a down-hole pressure and temperature transducer (DHPTT). This is a package that is located on the lowermost end of the production tubing (string) to give a continuous read-out of well pressure and temperature. Through the acquisition of temperature and pressure information from multiple wells, an operator can control a number of wells located in the same reservoir. FIG. 1 shows a typical sub sea layout with multiple well/drill centres.
The following is a description of a typical prior art “running” (i.e. installation) procedure.
FIGS. 2a to 2e show, schematically, the various stages of running culminating in a completed installation in which the well is being permanently monitored according to a previously considered method.
In FIG. 2a the well has been constructed with the wellhead 10 prominent above the seabed 12. It has been installed with a mechanical actuator 14, attached on the side of the wellhead 10, which will subsequently be used to make an electrical connection to a down-hole cable (not shown) inside the well head by penetrating through the wellhead to accommodate an electrical “wet mate” connector in a radial direction through the side of the well head. This procedure is described in detail in U.S. Pat. No. 5,558,532 (Hopper). A signal cable 16 leads from the mechanical actuator 14 to a dynamically positioned floating semi-submersible platform 18 on the surface for eventual monitoring of a down-hole device after installation.
In FIG. 2b, which depicts the next stage of the process, a tubular string 20 is lowered through the floating semi-submersible platform in short screwed-together sections. Any electronic sensors or devices are conveyed to the seabed well on this tubular string. A down-hole monitoring cable (not shown in the figure) is attached to the devices and is located within the tubular string as the assembly is lowered to the seabed. Once the calculated length of tubes is installed to fit the well depth, a ‘tubing hanger’ 22 is attached to the tubes to allow the installation to hang from a profile 26 in the sea bed known in the industry, on account of its shape, as a “Christmas tree” (a steel housing that remains at the well head and allows tubes to hang and valves to be attached). The tubing hanger 22 and tubing 20 are conveyed to the “tree” at sea floor by a releasable latch known as a tubing hanger running tool 24. This is attached to a profile in the tubing hanger 22 and the entire assembly (string) is then conveyed to the sea floor by adding lengths of screwed tubing until the tubing hanger reaches and engages the tree. This is a standard procedure.
FIG. 2c shows the running tool after it has just been disconnected. The running tubes can now be retrieved to the surface.
FIG. 2d depicts a remote-operated vehicle (ROV) 28 mechanically turning the actuator 14 that pushes forward the wet mate horizontal connector to make a permanent connection to the down-hole devices via the down-hole cable.
FIG. 2e shows the final configuration when the well is complete and the permanent monitoring cable 16 is commissioned to a final vessel or semi-floating work platform.
In view of the high costs of repair work in the deep sea environment, as outlined earlier, there is a strong incentive to monitor equipment to check that it is functioning during installation, in order to avoid the need for a costly work over. Thus, a device that is developed as part of the installed sub sea well head that allows electrical signals to be switched from monitoring whilst running (i.e. whilst installing) to permanent monitoring (i.e. after installation) is desirable, especially in the arduous sub sea environment.
One disadvantage of the prior system, as outlined above with reference to FIG. 2, is that the process does not permit monitoring of the down-hole device during installation (running).